Method of retarding erosion of gun barrels



Feb. 21, 195% N. H. SMITH 2,493,052

METHOD OF RETARDING EROSION 0F GUN BARRELS Filed Jan. 5, 1946 RoundsFired 0 60 I20 I60 240 360 420 480 540 600 660 720 780 840 900 I020Rounds INVENTOR M6 HAL? 5M! W BY ATTORNEY Patented F eb. 21, 1950 METHODOF 'RETARDING EROSION OF GUN BARRELS Nicol H. Smith, Philadelphia, Pa.,assignor to the United States of America as represented by the Secretaryof the Navy Application January 5, 1946, Serial No. 639,461

1 Claim. (01. 89-14) This invention relates to ordnance of the typewhich includes a rifled gun adapted to fire ammunition having a,rotating band, or equivalent structure, coacting with the gun rifling tocause rotation of the issuing projectile for stabilizing it in flight.More particularly, the invention relates to a novel method of prolongingthe useful life of such guns in their use with ammunition of the typedescribed.

An object of my invention is the provision of a simple, direct andthoroughly practical method for retarding the erosion of guns.

Another object of my invention is the provision of a method forretarding gun erosion which may be successfully practiced in connectionwith guns of a wide variety of sizes for automatic or single shot fireat high or low velocities.

As conducive to a clearer understanding of certain features of myinvention, it may be noted that gun barrel erosion is a complexphenomenon involving a combination of numerous effects, predominantlythermal efiects, chemical and metallurgical effects, and mechanicaleffects, all of which contribute as the result of gun use to shorteningthe useful life of the gun. The individual intensities of the variouseffects are determined by such factors as barrel material, type andquantity of powder and type of projectile employed, rate of fire, andvelocity of projectile.

As the result of high temperatures reached by powder gases, a thin layerof metal near the bore surface of the gun is rapidly heated to atemperature which under certain circumstances may exceed melting rangeof the metal. Even if the bore surface is not melted, the temperaturesreached are such as to reduce the strength of the steel and facilitateits removal. In cases where gas leakage occurs, the material may besimultaneously melted and carried away by scouring action of theescaping gas.

At the high temperatures attained by the powder gases in the gun, theproducts of the powder explosion are chemically active. Furthermore, theraising of the bore surface temperature increases the reactivity of thegun material. As a result, there is a very complex set of reactionsbetween the gun material and the powder gases. .The importance ofchemistry in the erosion process has not as yet been fully evaluated,but the existence of a considerable variety of reaction products hasbeen established.

. In firing a gun, the temperature attained by the steel bore surface iswell above the alphagamma transformation point. A transformed surfacelayer frequently develops to some thou- .resides in the provision of amethod for improvsandths of an inch thickness which differs markedly inits metallurgical characteristics from the underlying steel. This layerhas lower strength, probably a lower melting point due to chemicaleffects, and is less resistant to erosion than the underlying steel.

There are several mechanical effects which are known to contribute tothe erosion process. In

the first place, simple frictional or abrasive wear is a factor theimportance of which depends on both design and materials. When thesurface of a steel projectile bears on a bore surface of steel, gallingmay occur in addition to abrasive wear, with the consequent removal ofsmall masses of metal from the bore surface. Both the abrasive wear andthe galling tendency are affected by the'surface temperature attained inthe bore. This temperature, at the point of rubbing,

is influenced by the instantaneous heating effect caused by friction.

It is also apparent that plastic displacement of metal adjacent the gunbore surface, by hammer- .ing band is accompanied by an upwarddisplacement of theland surface ahead of it. In effect,

a waveconsisting of a depression of the land material near the origin ofrifling and'a raising of material in front of this region proceeds downthebore as firing progresses.

Accordingly, another object of. my invention ing the resistance of gunsto plastic deformation .and erosion normally produced by thermal,chemical and metallurgical, and mechanical effects combined.

These and other objects of the invention may be better understood byreference to the following detailed description and the accompanyingdrawing, in which 1 Fig. 1 is an enlarged cross-sectional view of partof a rifled gun with a projectile therein, illustrating the relation ofthe rifling to the rotating band on the projectile, and

Figs. 2 and 3 are graphs illustrating the comparison of the gun life inthe practice of the present invention with the gun life in the practiceof prior methods, with respect to changes in projectile velocity andchanges in gas pressure in the gun, respectively.

Referring now more particularly to the practice of my invention, Iprovide a gun of any desired size, to be employed for automatic orsingle shot fire at high low velocity, having a bore surface lining ofrefractory metal, which may be chromium or other erosion resistantmaterial, such as molybdenum, tungsten or tantalum in normal or inhardened condition, which is capable of withstanding gun temperatureeffects and chemical attack. The protective lining presents a grooved orrifled surface as a result, for example, of depositing the protectivemetal onto the bore surface of a rifled barrel, or-ofrifling the linerbefore or after it is placed in the barrel. According to my invention, Iprovide against rapid wear of this protective metal, due to mechanicaleffects such as friction, abrasion and galling, by bringing intoimmediate contact with the protective lining in the process of firing,only projectiles having a pre-engraved rotating hand or equivalentstructure, so that the engraving is not performed by the action of therifiing. The protective lining preferably is employed in suchthicknesses as to insulate the underlying gun steel from heat ofexplosion to a measure which will prevent the, formation of atransformed layer in the steel.

' As illustrative of the practice of my invention, I provide a gunbarrel l of steel which is hellcally grooved or rifled to provideinternal lands H and grooves l2, the gun being electroplated at the boresurface with a thin adherent deposit [3 of substantially pure chromiumdeposited as by a standard commercial or U. S. Navy plating process, orby the method disclosed in a patent to Olin et al., No. 1,886,218,granted Nov. 1, 1932, for Gun barrel and process of finishing the same.The plating I3 is preferably though not necessarily equal to or greaterthan established minimum value of approximately 0.003 inch. For longestlife, the minimum thickness of the chromium I3 is important inpreventing the formation of transformed or alteredlayers in theunderlying gun steel [0 normally caused by sudden heating in combinationwith chemical attack and subsequent cooling, and prevents undercuttingand stripping and lessens cracking of the chromium. The lining l3 itselfis more. reliably adherent and will endure longer when the gun steel I0,beneath it does not transform. It will be understood that it may benecessary to vary somewhat the effective minimum thicknessof the platingl3, depending upon such factors as the size of barrel l0, character orrate of fire, type of powder, and muzzle velocity of projectile to beemployed with the plated barrel.

Although an improvement in the life of guns is brought about by chromiumplatingsor platings of other erosion resistant material, it has beenfound that when a substantial number of rounds have been fired usinstandard ammunition, the stresses due to the usual engraving of theprojectile by the rifling cause such platings to be destroyedmechanically. For example, a gun having a bore which is chromium plated,as described, and from which ball bullets, are fired, shows removal ofchromium from the forcing cone, obliteration of the origin of rifiing,and removal of metal from the driving edges of the lands. It thus canbe. seen that before long the plating [3 will be removed mechanically.The

removal occurs predominantly at the breech end of the gun where erosiveinfluences of heat and chemical attack are most severe, leaving theunderlying steel l0 inadequately protected.

I have discovered that much can be added to the useful life of gunsbore-plated with chromium or other erosion-resistant material, byemploying pre-engraved projectiles in such guns. More particularly, theprojectile I4 is pre-engraved, as shown at l5, so that the engraving isnot performed in the usual manner by the action of the rifiing lll2 onthe projectile as it passes through the gun bore. It will be understoodthat the projectile I4 may be pre-engraved directly onthe, core itself,as in the case of small caliber bullets which do not have a rotatingband as such, or the pie-engraving may be performed on a rotating bandIE on the projectile. The preengraving may be effected in any desiredmanner, as by machining the projectile. The pre-engraved projectiles Mare preferably so dimensioned as to form a somewhat loose fit with theplating l3 or to provide a small clearance, as for example, 0.006 inchon the diameter across gun lands. The loose projectiles further preventmechanical effects on the chromium or other plate.

A phenomenal increase in-the erosion lifeof a caliber 0.50 machine gunbarrel, fired under hypervelocity conditions, was obtained by the use ofpre-engraved bullets in loose fitwithabore surface plate of chromiumhaving about 0.003 inch thickness to prevent thermal alteration of theunderlying steel. The height of plated lands from the bottoms oftheplated grooves was approximately 0.010 inch, which is somewhatgreater than that employed with respect to conventional caliber 0.50machine gun lands. On the other hand, the individual width of platedlands in the gun approximated 0.060 inch, while the width of groove inthe pre-engraved projectiles fired was about 0.071 inch. A clearancemaintained betwen tops and bottoms of interfitting lands and grooves ofbul let and. gun amounted to about 0.006 inch on the diameter. Theclearance between lands of the bullet and grooves of the gun was madeslightly gerater than the clearance between grooves of the bullet and'landsof the gun, which as a general principle gives improved accuracyof fire. With these conditions, the plate was just beginning to chip offthe lands when testing had been discontinued after 440 rounds. There wasonly a slight decrease in velocity. A similar barrel, with anunprotected steel bore surface, eroded to such an extent when copperbanded projectiles without pre-engraving were fired with the same powdercharge as used above, that velocity decreased 200 feet per second afteronly rounds.

Referring now to Fig. 2 I have shown a graph illustrating a comparisonof the projectile velocity changes occurring in the use of a gun inaccordance with prior practices, with the projectile velocity changesoccurring in the use of a gun in accordance with the present invention,As there shown, the ordinates represent projectile velocity changes infeet per second, and the abscissas represent the number of projectilerounds fired from the gun. In Fig. 3, I have shown a graph similar tothat in Fig. 2, except that the comparison is based upon pressurechanges occurring in the gun barrel, the ordinates representing gunpressure changes instead of projectile velocity changes, On Figs. 2 and3, theline (a) represents the changes in a gun having an unplated boreand using standardammunition which is not preengraved; the line (1))represents the changes in a gun having an unplated bore but usingpreengraved ammunition; the line (0) represents the changes in a gunhaving its bore chromium plated to a thickness of .005" and usingstandard ammunition which has not been pre-engraved; and the line (:1)represents the changes in the gun chr0- mium plated as described aboveand using preengraved ammunition, in accordance with the invention. Thelines (a), (b), (c) and (d) were derived from plots of values obtainedfrom the actual firing of a standard type gun using standard typeammunition, under the conditions specified.

From an examination of Figs. 2 and 3, it will be seen that theconditions represented by the lines (b) and (0) improve somewhat theuseful life of the gun over that which is normally obtained under thestandard conditions represented by line (a). However, in the use of thegun according to the present invention, as represented by line (d), theimprovement in the useful gun life is not merely an approximation of thealgebraic sums of the improvements represented by lines (b) and (c), asmight be expected, but is many times as great as the improvementsrepresented by lines (17) and (0). Moreover, in the practice of the newmethod, the projectile velocity actually increases at first and remainsgreater than the normal velocity for a substantial number of rounds (180as shown), and the pressure in the gun actually increases at first andremains greater than the normal pressure for a much longed period of use(780 rounds as shown).

Numerous results of other tests show that erosion resistant refractorymetal linings on the bore surface of guns contribute much to the usefullife of a gun, particularly when provided in such thickness as toprevent heat alteration of the underlying steel and when protectedagainst mechanical wear and distortion normally caused by engravingstresses. Chromium, in particular, I find, is resistant to thermal andmetallurgical efiects in guns. It is protected from wearing excessivelydue to mechanical influences when pre-engraved projectiles are used.Thus, it will be seen that in the present invention there is provided amethod of prolonging the useful life of guns, in which the objectshereinbefore noted together with many thoroughly practical advantagesare successfully achieved. It will be observed that the methodalleviates many dilficulties encountered in repair and replacement oferoded guns and the associated problems of production andtransportation.

It will be understood that while the use of chromium or other protectivemetal or metal alloy linings, such as plates, coatings or claddingswhich cover the entire bore surface of the gun, are preferred,relatively short linings covering the breech zone which is exposed tomost severe thermal, chemical and mechanical attack may also be usedwith good results. The linings preferably are deposited electrolyticallyor by other depositing methods as from the metal carbonyl, but it willbe understood that certain alternatives such as claddings, or removablesleeves may be employed where practical. It will also be understood thatthe linings, such as the platings, claddings, sleeves and the like, maybe either composite or of all one metal.

In way of illustration, my method may be practiced in connection withguns which have been modified as by electropolishing or machining fromstandard finished dimensions to receive the necessar lining as byelectroplating back to finished dimensions, as well as with respect toguns which have been made to finished dimensions initially-with thelining included.

In the following claims, it is to be understood that the expressionplating the rifled bore of the gun is intended to denote theapplication, by plating, electroplating, coating, or other means, of arelatively thin layer of erosion-resistant material to the rifled bore,whether the rifled bore is in a liner or is directly in the gun barrelproper.

As many possible embodiments of my invention may be made, and as manychanges may be made in the embodiments hereinbefore set forth, it willbe understood that the matter described herein is to be interpreted asillustrative and not as a limitation.

I claim:

A method of prolonging the useful life of a rifled gun in the firing ofprojectiles, which comprises plating the rifled bore of the gun with anerosion-resistant material, and ore-engraving said projectiles toprovide grooves and lands which fit loosely with the lands and grooves,respectively, on the plated, rifled bore surface of the gun, and withthe clearance between lands of the projectile and grooves of the gunslightly greater than the clearance between grooves of the projectileand lands of the gun.

NICOL H. SMITH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,659,625 C'owan Feb. 21, 19281,777,519 Flowers Oct. 7, 1930 1,886,218 Olin et a1 Nov. 1, 1932

